In the fields of anaesthesia and pain management the need to accurately position a medical needle within the body is frequently encountered. Such needles may be used in the delivery of fluid, such as an anaesthetic reagent, wherein the position of the delivery of the fluid is important in achieving an optimal effect or in avoiding potentially damaging side effects. Likewise, medical needles that are used to perform a biopsy should be accurately positioned in order to ensure that the correct tissue sample is taken from the body.
Medical photonic needles with optical fibers inside are known. Such needles allow optical probing of biological tissue, e.g. for positioning or for other purposes and typically have one light-supplying fiber and one light-receiving fiber. In alternative configurations a single fiber performs both light-supplying and light-receiving functions.
In the original concept of the photonic needle, a needle with straight ended fibers at the tip was used as disclosed in patent application WO2013054254A.
Medical photonic needles having optical fibers in which the end surface of the optical fibers are beveled, are also known. See for example “Epidural needle with embedded optical fibers for spectroscopic differentiation of tissue: ex vivo feasibility study” by Desjardins et al, Biomed Opt Express. 2011 Jun. 1; 2(6): 1452-1461. In this publication the beveled surface of the needle was angled at 20 degrees with respect to the needle axis. Three optical fibers were embedded in a cannula with epoxy and polished so that their distal ends were flush with the beveled surface. Such a small bevel angle provides a sharp point that facilitates insertion into a body.
Medical photonic needles in general however, suffer from a number of problems.
The optical performance of medical photonic needles having both straight-cut and beveled optical fiber(s) is conventionally calibrated in air by placing a reference optical surface in front of the light supplying fiber such that light is directed back into the light receiving fiber. The optical performance is determined on the basis of the proportion of supplied light that is subsequently received, and on the basis of the optical properties of the reference optical surface.
A problem resulting from the usual in-air calibration is that the amount of internal reflection of light within the optical fiber at the fiber tip is dependent upon the refractive index of the optical medium that is in contact with the fiber tip. Since this optical medium is air, having a refractive index of unity, during calibration, the differing refractive index of tissue during use of the medical needle, means that the in-air calibration data has limited relevance. In order to resolve this issue is it also known to calibrate the optical fibers of such medical needles in for example liquid media having a refractive index that is closer-matched to that of tissue.
The present invention seeks to improve the robustness of the optical calibration of such photonic needles.